1. Field of the Invention
The invention generally relates to computer aided semiconductor design methods and semiconductor designs and structures which are produced using those methods. In particular, the invention provides methods for evaluating minority carrier transmission for noise and latchup in a semiconductor chip.
2. Background of the Invention
As the electronic components and internal structures of integrated circuits are becoming smaller and smaller, they are being placed closer together on the substrate surface. As a result, the electronic components are, unfortunately, becoming more and more susceptible to impairment and/or destruction due to “latchup”. Latchup occurs when a pnpn structure transitions from a low current, high voltage state to a high current, low voltage state through a negative resistance region, i.e. when an S-Type I-V (current/voltage) characteristic is formed. Latchup can occur anywhere there is a PNPN structure and excess minority carriers. PNPN structures are ubiquitous in semiconductor devices, particularly CMOS, and excess minority carriers (e.g. electrons) can be developed from many sources.
Latchup is typically understood as occurring within a pnpn structure, such as a silicon controlled rectifier (SCR) structure. These pnpn structures can be either intentionally designed or unintentionally formed between structures. Hence, latchup conditions can occur within peripheral circuits or internal circuits, within one circuit (intra-circuit) or between multiple circuits (inter-circuit).
Latchup is typically initiated by the PNPN structure forming an equivalent of a circuit having cross-coupled pnp and npn transistors. With the base and collector regions being cross-coupled, current flows from one device leading to the initiation of conduction of the second device (“regenerative feedback”). These pnp and npn elements can be any diffusions or implanted regions of other circuit elements (e.g. P-channel Metal Oxide Semiconductor Field Effect Transistors (MOSFETs), NChannel MOSFETs, resistors, etc.) or actual pnp and npn bipolar transistors. In Complementary Metal Oxide Semiconductors (CMOS), the pnpn structure can be formed with a p-diffusion in an nwell, and in an ndiffusion in a p-substrate or p-well (parasitic pnpn). In this case, the well and substrate regions are inherently involved in the latchup current exchange between regions.
The condition for triggering latchup is a function of the current gain of the pnp and npn transistors, and the resistance between the emitter and the base regions. This inherently involves the well and substrate regions. The likelihood or sensitivity of a particular pnpn structure to excess minority carriers and latchup is a function of spacings (e.g. base width of the npn and base width of the pnp), bipolar current gain of the transistors, substrate resistance and spacings, the well resistance and spacings, and isolation regions.
Excess minority carriers and latchup can be initiated from internal or external stimuli. Latchup is known to occur from single event upsets (SEUs). SEUs include terrestrial emissions from nuclear processes and cosmic ray events, as well as events in space environments. For example, terrestrial emissions from radioactive events, such as alpha particles and other radioactive decay emissions, can lead to latchup in semiconductors. Cosmic rays, a number of which are known to enter the earth's atmosphere, include proton, neutron, and gamma emissions, and are also known to cause latchup.
Latchup can also occur as a result of voltage or current pulses in power supply lines (VDD and substrate ground VSS). For example, latchup can be initiated by a negative transient on the VDD power rail which can lead to a forward biasing of all the ndiffusions and nwell structures and electron injection throughout the semiconductor chip substrate. This produces a “sea of electrons” injected in the chip substrate. Equivalently, a positive transient on the VSS power rail can lead to hole injection, and forward biasing of the substrate-well junction providing a “sea of holes” event. Further, latchup can also occur from a stimulus from a minority carrier to the well or substrate external to the region of the thyristor structure. Excess numbers of minority or majority carriers less than sufficient to cause latchup can nevertheless appear in signals as noise.
In both internal circuits and peripheral circuitry, latchup is a concern. Latchup also occurs as the result of interactions of Electrostatic Discharge (ESD) devices, I/O off-chip drivers, and adjacent circuitry initiated in the substrate from overshoot and undershoot phenomenon. Simultaneous switching of circuitry, where overshoot and undershoot injection occurs, may lead to carrier injection into the substrate which causes both noise injections and latchup conditions. Supporting elements in these circuits, such as pass transistors, resistor elements, test functions, over-voltage dielectric limiting circuiting, bleed resistors, keeper networks, and other elements can also contribute to injection into the substrate. Electrostatic discharge (ESD) elements connected to the input pad (including MOSFETs, pnpn Semiconductgor-Controlled Rectifier (SCR, i.e. thyristor), ESD structures, p+/nwell diodes, nwell-to-substrate diodes, n+ diffusion diodes, and other ESD circuits) may also contribute to noise injection and latchup.
Latchup is a higher risk in regions where the incoming voltage exceeds the native power supply voltage of the chip. Circuits, input pins or power rails which receive voltages that far exceed the native voltage, are sources for latchup concern. Examples include regions of semiconductor chips that provide programmable Non-Volatile Random Access Memory (NVRAM) power pins, mixed voltage off-chip driver circuits, mixed voltage ESD networks, mixed voltage receiver networks, and mixed voltage ESD power clamps are all sources for latchup concern. Placement of circuit blocks and circuit functions with a high density of pnpn devices, or weakly robust networks that are near the high voltage supply, are especially vulnerable to trigger latchup in a chip.
Many different stimuli and network functions exist in a semiconductor environment. Peripheral circuits include ESD networks, transmitter and receiver networks, system clocks, phase lock loops, capacitors, decoupling capacitors and fill shapes. Internal circuits include Dynamic Random Access Memory (DRAM) memory, Static Random Access Memory (SRAM) memory, gate arrays and logic circuitry. These regions form shapes both in and outside the silicon substrate. The regions inside the silicon substrate are those which allow electrons to 1) escape or 2) to be trapped. At any closed circuit domain region, the sum of probability of trapping and the probability of escaping is unity.P(trapping)+P(escaping)=1
The trapping probability is defined by quantifying the spatial regime at which the electron is trapped either by collection from an n-diffusion, n-well, or n-resistor shape in a p-silicon substrate. The trapping probability also includes the recombination of an electron (e.g. minority carrier) with a majority carrier inside the domain or defined region. Trapping probability is also influenced by trench structures such as deep trench (DT), trench isolation (TI) and shallow trench isolation (STI) via surface recombination, or reflection of the carrier into the space preventing transmission to outside of the physical domain.
A second perspective is that electrons injected into the substrate are either 1) collected at a junction region, or 2) recombine in the bulk or at an interface. Hence, the probability that an electron is collected plus the probability that an electron recombines equals unity.P(recombine)+P(collected)=1
The probability of escape is the probability that an electron is collected or recombines outside the domain of interest. From this perspective, the probability of a guard ring collection is the current measured at an additional ring outside of the guard ring and the p+ substrate contact outside of the guard ring normalized to the injection current. The probability that an electron escapes from a guard ring or series of guard rings, is the current measured at an additional ring outside of the guard rings and the p+ substrate contact outside of the guard ring, normalized by the injection current. The efficiency of the ratio of captured electrons in the guard ring structure to the injected current is a measure of the guard ring efficiency. When the electron current outside of the guard ring is small compared to the current collected on an n-well diffusion, then the escaped collected current normalized to the injected current is also a metric for evaluation of the guard ring effectiveness (of lack thereof) or a measure of the probability of escape. Therefore, the transmission of minority carriers is related to the escape probability out of a physical region or boundary.
Protection against spurious transmission of minority carriers is generally avoided by guard rings or other shapes and structures in or on the silicon. However, such shapes and structures have differing levels of effectiveness in capturing or reflecting minority carriers. Further, the design of guard rings in the chip floor-plan has the undesirable effect of taking up additional space, already at a premium in the floor-plan.
The prior art has thus far failed to provide: a method to determine the probability of escape (or the probability of transmission outside a domain) in a chip environment; a method to determine the transmission probability of minority carriers from a first point in a semiconductor chip to a second point in a semiconductor chip; a method which can quantify the minority carriers that reach a physical region or device by evaluation of the transmission factors from the injection source to the collecting region on interest; a method, apparatus, or structure that predicts and improves the latchup tolerance and guard ring efficiency in a complex semiconductor environment by addressing global as well as local interactions; a method, apparatus, or structure that predicts latchup within a circuit function and between circuit functions in a complex semiconductor chip; a method, apparatus, or structure that predicts and improves latchup tolerance by addressing global and local temperature and how it influences the transmission probability with a physical domain, both locally and globally; a method that evaluates periodic “unit cells” and evaluates their transmission probability for evaluation of probability of a minority carrier transmitting through the unit cell; and a method that evaluates periodic “unit cells” and evaluates the global transmission probability for evaluation of probability of a minority carrier transmitting through a periodic array of unit cells or a function block of unit cells. Such methodology would be useful in order to predict which areas of a chip would be more or less susceptible to noise and latchup, and would thus allow for improved design of chip floorplans that are resistant to noise and latchup.
In a preferred embodiment of the invention, a computer aided design apparatus is provided. The apparatus comprises a graphics generator, a schematic generator, and a graphical unit interface containing a parameterized cell. The parameterized cell is defined by the graphics generator and the schematic generator, and contains transmission, absorption, and reflection parameters corresponding to the parameterized cell.